The present invention relates to an ion trap, a mass spectrometer, a method of trapping ions and a method of mass spectrometry.
Ion trapping techniques are well established in the field of mass spectrometry. Commercially available three dimensional Paul ion traps and linear geometry ion traps (LIT) based upon a quadrupole rod structure provide a powerful and relatively inexpensive tool for many types of mass spectrometry. Ions are trapped with these devices by inhomogeneous fields modulated at radio frequencies (RF confinement). DC trapping potentials may also be used. Mass selective axial or radial ejection may be achieved by a variety of different techniques. However, to varying degrees, conventional commercial ion traps suffer from limited dynamic range due to the onset of space charge saturation effects at high ion population density.
Space charge saturation in an analytical ion trap is characterised by a loss in analytical performance such as mass resolution, mass measurement precision or accuracy and precision of quantitation and in spectrum dynamic range.
Space charge saturation effects in a conventional commercial linear quadrupole ion trap can become significant for ion populations comprising approximately 30,000 charges. However, in normal operation, linear quadrupole ion traps are capable of trapping much larger ion populations even though analytical performance will be compromised. The total charge capacity of such an ion trap may be several orders of magnitude higher than the space charge limit for acceptable analytical performance.
Various methods are known which attempt to control or limit the total charge entering an analytical ion trap. The conventional methods generally require a pre-scan in which a measurement is made of the composition of the incoming ion beam over a fixed period of time. The amount of signal recorded in the pre-scan is then used to estimate the time for which the incoming ion beam should be allowed to fill the analytical ion trap such that the population of ions does not exceed a target value. However, during the time taken to perform a pre-scan and to perform an analytical scan of the ion trap, incoming ions are lost and hence the duty cycle of the experiment and overall sensitivity is reduced.
In addition, during a pre-scan an estimate of the total charge is generally made from the amplitude of the detected signal. However, the amplitude response of the detector may not be linear for ions having differing charge states and masses. Therefore, for populations including highly charged species the total charge may be underestimated using conventional techniques. The level at which space charge can compromise performance is generally dependent upon the total charge in the ion trap and not necessarily upon the number of ions in the ion trap.
It is desired to provide an improved ion trapping arrangement.